1. Field of the Invention
The present invention relates to a reversible electrode containing an active transition metal and a matrix polymer with a basic functional group, which has affinity to the transition metal species. This invention also relates to a secondary polymer battery containing said electrode as a positive electrode (hereinafter referred to as a xe2x80x9ccathodexe2x80x9d) with high energy density and good reversibility.
2. Discussion of Related Art
A battery is one of the essential components of electronic equipment widely used in modern life. Especially, the development of electronic communication and the computer brought a rapid and wide distribution of portable devices. And the secondary battery that is repeatedly usable by recharge is highly demanded as an energy source of the portable apparatus. Therefore, these kinds of small secondary batteries need to be small volume, light-weight, and longer lasting upon repeated cycle of charge and discharge, to be suitable for hand-carrying applications. In order to meet these requirements, electrode material with high energy density should be provided.
There have been much research efforts to utilize high energy density of transition metal as a cathode active material in a secondary battery. For example, U.S. Pat. Nos. 4,546,055, 4,795,685, 4,992,345, 4,288,506 and 4,797,333 describe a secondary battery comprising an alkali metal anode, a molten sodium aluminum halide (NaAIXn) electrolyte, a solid electrolyte separator such as xcex2-alumina, and a cathode, wherein the cathode active material is a halide of a transition metal, such as Fe, Ni, Co, and Cr.
However, there are several disadvantages associated with these batteries, including high operation temperature over 70xc2x0 C. required to maintain molten state of sodium aluminum halide (NaAIXn) electrolyte and formation of corrosive metal chloride. Formation of an electrically insulating layer by a metal chloride was also pointed out in U.S. Pat. No. 5,283,135. And, the agglomeration of the metal particles causing a short cycle life and addition of the powder of a transition metal, such as Fe, Co and Mn, to nickel halide to prevent agglomeration were described in U.S. Pat. No. 5,573,873.
On a different aspect, as for a small secondary battery comprising a transition metal as the cathode active material, the U.S. Pat. Nos. 4,714,665 and 4,728,588 disclose a secondary battery consisting of three layers of polymer films. In order to increase the content of metal species in the electrode with minimum agglomeration, a large amount of organic plasticizer which dissolves active metal species of the formula MXa, wherein M is Li, Ca, Cu and Zn, and X is Cl, Br, and 1, was added. The U.S. Pat. No. 4,714,665 discloses a secondary battery consisting of polymer film electrodes which is light and flexible. Metal salts are dispersed in the same composition of polymer matrix and plasticizer as polymer electrolyte layer to provide high compatibility between electrode and electrolyte layers. And, the U.S. Pat. No. 4,728,588 discloses a secondary battery consisting of a single polymer sheet which contains a metal salt as the electrode active material, polymer matrix and plasticizer.
The content of the metal salt in the electrode prepared in this manner highly depends upon the content of the plasticizer. However, the content of the plasticizer changes due to the volatility and is limited to the minimum fluidity required to maintain the physical shape of the electrode. Even though the concentration of metal species was suggested to be 5 to 30%, the capacity of the electrode that is directly derived from the content of active metal species is limited by the amount of plasticizer in the electrode. In the U.S. Pat. Nos. 4,620,026 and 4,654,390, the use of an ester compound as a plasticizer is described, and this ester compound has low volatility and a good low-temperature characteristic.
As mentioned above, there are many problematic aspects in using a transition metal as a cathode active material for a secondary battery.
The present invention is directed to a cathode material composition with a high capacity that substantially obviates one or more of the aforementioned problems of said prior arts which use the transition metal compound as the cathode active material.
It is therefore an object of the present invention to provide a cathode composition including one or more transition metal compounds selected among transition metal or a transition metal salt and a matrix polymer which has a basic functional group with an affinity for the transition metal species.
It is another object of the present invention to provide a cathode composition, wherein a high content of transition metal compound is homogeneously dispersed in a matrix polymer which has a basic functional group with an affinity for the transition metal species.
It is still another object of the invention to provide a cathode including a transition metal/matrix polymer composition, which has a high capacity and a high charging/discharging efficiency upon repeated cycles of charge and discharge.
It is a further object of the invention to provide a secondary polymer battery of the all-solid type, which does not impose any problem relating to usage of a liquid component and has a flexibility in the design of the battery, such as mini-cell or thin-cell according to the various applications.
Additional features and advantages of the present invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention.
The objectives and other advantages of the present invention will be realized and attained by the structure of the invention particularly pointed out in the written description and claims hereof as well as the appended drawings.
In order to achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a polymer secondary battery of the present invention comprises
(a) a cathode prepared by casting on an electrically conductive collector with a high capacity cathode active material composition including i) one or more transition metal compounds selected among transition metal and transition metal salt of the formula MXn, wherein M is a transition metal, X is a halogen negative ion, sulfur atom or conjugate ion of acid, and n is an integral number between 1 to 6; ii) a matrix polymer which has a basic functional group with affinity for the transition metal species;
(b) a polymer electrolyte containing a lithium salt; and
(c) an anode including the material selected among a lithium metal, a lithium alloy, and a lithium intercalation material such as carbon.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the present invention as claimed.